1. Field of the Invention
The present invention relates to a roller supporting structure for e.g., a thread feed roller or the like, which utilizes a dynamic pressure as a support means.
2. Description of the Prior Art
FIG. 3 shows a conventional roller supporting structure. In the supporting structure of FIG. 3, grooves 53 for generating a dynamic pressure are formed in V-shape on an outer peripheral surface of an intermediate shaft part 52a of a larger diameter of a shaft 52 which part supports a roller 51. Retaining plates 55, 56 are fixed to the shaft 52 in a manner to sandwich the roller 51 therebetween from both ends in the axial direction. The roller 51 has permanent magnets 59, 60 fixed to its end faces. On the other hand, permanent magnets 57, 58 are fixedly mounted at roller-facing end faces of the respective retaining plates 55, 56 confronting the end faces of the roller 51. The opposed permanent magnets 57 and 59, 58 and 60 of the retaining plates 55, 56 and the roller 51 are of the same polarity.
As the roller 51 is rotated, a dynamic pressure is generated in an ambient fluid between the shaft 52 and the roller 51 by the grooves 53 of the shaft 52, so that the roller 51 is supported by the dynamic pressure of the fluid in the radial direction. Moreover, a repulsive force between the permanent magnets of the same polarity 57 and 59, 58, 60 supports the roller 51 in the axial direction.
Another conventional roller supporting structure is indicated in FIG. 4. In this supporting structure, grooves 63 for generating a dynamic pressure are formed in V-shape on an outer peripheral surface of an intermediate shaft part 62a, a larger diameter part, of a shaft 62 which part supports a roller 61 as in the roller supporting structure of FIG. 3. The roller 61 is sandwiched between the retaining plates 65 and 66 from both ends thereof in the axial direction with the retaining plates at predetermined distances from the end faces of roller 61. The retaining plates 65, 66 fixed to the shaft 61 also have grooves 67, 68 formed at their roller-side end faces confronting the end faces of the roller 61.
In the roller supporting structure of FIG. 4, when the roller 61 is rotated, the grooves 63 of the shaft 62 cause an ambient fluid between the shaft 62 and roller 61 to have a dynamic pressure and thereby to support the roller 61 in the radial direction with the dynamic pressure. Moreover, the retaining plates 65 and 66 are sufficiently close to the ends of the roller 61 so that grooves 67, 68 of the retaining plates 65, 66 generate a dynamic pressure in an ambient fluid between the roller 61 and the retaining plates 65, 66. Thus the roller 61 is also supported by the dynamic pressure in the axial direction.
The above supporting structures have the following drawbacks. In the former structure using permanent magnets for supporting the roller in the axial direction, the repulsive forces of the permanent magnets 57, 58 and 59, 60 of the retaining plates 55, 56 and the roller 51 cause forces pressing the roller 51 in the opposite axial directions even when the roller 51 is not rotated. Therefore, when there is imbalance between the repulsive forces of the permanent magnets at opposite ends, the imbalance causes a bending moment acting on the roller 51. The bending moment increases a frictional force between the inner peripheral surface of the roller 51 and the outer peripheral surface of the intermediate shaft part 52a of the shaft 52 when the roller 51 starts to rotate. Therefore, a large torque is required to start the roller 51.
Meanwhile, according to the latter arrangement, since it is necessary that the predetermined distance `s` be approximately several .mu.m between each retaining plate 65, 66 and the roller 61, squareness between each retaining plate 65, 66 and the shaft 62 and flatness of the opposed end faces of the roller 61 and the retaining plates 65, 66 must be controlled with high accuracy. Therefore, the component parts, especially the roller 61 and the retaining plates 65, 66, are required to be worked and assembled highly accurately.